Breast cysts are fluid-filled sacs that can develop in the breast tissue. They are usually benign, meaning they are not cancerous, and can vary in size. Many women may have breast cysts without even knowing it, as they often do not cause any symptoms. These cysts can occur at any age but are most common in women between ages 35 and 50. They may feel like a smooth lump and can sometimes cause discomfort or tenderness, especially before a menstrual period. Regular breast exams and mammograms can help monitor any changes in breast health.